The present disclosure is related to an apparatus and method for determining aiming corrections for improving the accuracy of small arms. More specifically, the present disclosure is related to a manual calculator used to adjust the aiming point of a small arms weapon.
Several factors are known to affect the accuracy of small arms weapons including rifles used to fire at targets at ranges of greater than 1000 meters. For example, weapons up to and including 50 caliber rifles are used by the United States military to strike strategic targets at ranges reaching and exceeding 1500 meters. Proper aiming of a small arms weapon requires a user to adjust for the particular weapon and ammunition combination. The particular weapon and ammunition combination will result in a particular muzzle velocity, twist rate, ballistic coefficient for the bullet and spin drift which will result in a specific trajectory for the bullet under ideal conditions.
Other factors also influence the proper choice of aim point when a small arms weapon is being fired in the field. Atmospheric conditions including temperature, barometric pressure, and relative humidity affect the flight characteristics of a bullet. Ballisticians and weapons users often apply a factor known as the density altitude to adjust for the atmospheric conditions over a particular firing range. The density altitude is normally expressed in feet or thousands of feet and can be determined by adjusting the actual altitude above sea level based on the temperature at the actual altitude as defined by International Civil Aviation Organization (ICAO) Standard or by the Standard Metro developed by the US Army in 1905.
Another factor which affects the choice of the aim point is the target distance. At extreme distances, such as greater than 1500 meters, a Coriolis effect may impact the selection of the proper aim point. In most instances, the Coriolis effect is of such insignificance that it can be ignored in most instances, but it must be noted dependant on where you are on the earth and at what vector you will be firing. Coriolus functions much like spin drift but is tied to the relationship of the Earth's rotation and the amount of the Earth has moved during the time of fight of the bullet. This calculation is highly dependent on which hemisphere of the earth you are in as well as your longitude and latitude on the earth and the vector the bullet will be traveling. The above results in a correction to your aiming point based on these conditions. However, the target distance does relate to the effect of gravity on the bullet as it travels to the target. Compounding the effect of gravity in adjusting the aim point is the inclination or look angle between the firing point and the target. If the target is lower than the firing point, the ballistic trajectory of the bullet will have a shape that is significantly different from the shape of the trajectory for a bullet fired at a target having an elevation higher than the firing point. Thus, users adjust the aim point depending on the inclination angle as well as the average air density variation from the muzzle to the inclined or declined target. A cosine correction is used to compensate for the difference in the position of the firing point as well as adjustments for gravitational force variations and air density variations to the target and is dependent on both the target distance and the inclination angle as well as any additional modifications due to gravity from shooting up hill or downhill and from the bullet traveling thru high and lower air density resulting from shooting up or down.
In addition to the vertical adjustments described above, a user must adjust the point to correct for any wind that may be present over the firing range. It is known in the art to adjust an aim point based on a resultant wind vector that they weapon user calculates based on external factors observed over the firing range. The wind vector combined with the density altitude determines the horizontal deviation that will be experienced by a bullet in-flight over the range. Variations in ammunition result in variations in a ballistic coefficient of the bullet. The ballistic coefficient, expressed as a pressure, is a measure of the resistance applied to the bullet during flight. A higher ballistic coefficient, the lower the drag experienced by the bullet and subsequently reducing the effects of wind as well.
Finally, a user must consider the inherent variations of a particular weapon or around in making aim point adjustments. For example, a particular lot of ammunition may vary slightly in their performance, thereby affecting the accuracy at which the bullet can be aimed. The weapon user may also find that a particular weapon various slightly from the performance of a standardized weapon such that the user may need to adjust the either the ballistic coefficient or the muzzle velocity based on the particular weapon. The process of adjusting for the inherent variation in weapons and munitions used in the field is referred to as truing. Truing is the final consideration a weapon user must implement to maximize the accuracy at which the weapon may be fired.
Over time, ballisticians and weapons users have developed tools and calculators to be used by a weapon user to modify the aim point of a weapon to compensate for the various factors discussed above.